1. Field of the Invention
The invention relates to a nitride semiconductor laser device and to a method for improving its performance leading to an extension of its lifetime. In particular the method according to the invention relates to providing a window layer on the radiation emitting end face of the resonator. Such a layer increases durability of the nitride semiconductor laser device according to the invention.
2. Description of the Related Art
Lifetime of semiconductor laser devices depends primarily on quality of the optically active layers, and especially on quality of resonator mirrors of such layers. In the so-far used semiconductor laser diodes manufactured on the basis of semiconductors of the GaAs group, extension of the lifetime of a laser diode is achieved by forming special layers on resonator mirrors. These layers are used as antireflection layers or they form a window structure. Such a structure has been disclosed in Japanese patent application number 10-251577 published under a publication number 2000-082863.
The energy gap in a semiconductor window layers has to be wider than that in the active layer of semiconductor laser structures in order to increase durability of thus protected laser structures.
In nitride semiconductor lasers, resonator mirrors are obtained as a result of Reactive Ion Etching (RIE) or cleavage, and—due to energy gap narrowing—they absorb emitted radiation, which results in heat generation leading to impairment of the lifetime of over 100 mW laser diodes. Therefore, it was suggested that a window structure in nitride semiconductor lasers should be obtained by covering resonator end faces with the AlGaInN semiconductor layer (Japanese unexamined patent publication no. 249830/1995) or with a different layer, such as the AIN layer (Japanese unexamined patent publication no. 26442/2002).
According to the prior art technology, it is necessary to apply temperatures higher than 1000° C. to form the window layer made of monocrystalline gallium-containing nitride using growth methods from the gaseous phase, for example by the most commonly used Metallo-Organic Chemical Vapor Deposition (MOCVD) method. However, such high temperatures cause damage to the active layer formed of an indium-containing nitride semiconductor as used in nitride semiconductor lasers so far. On the other hand, when a nitride layer is formed by the currently known methods at temperature not causing damage to the active layer, the layer thus formed is amorphous. If the amorphous layer is used for forming a window structure, it brings about scattering of emitted light, due to which a laser beam becomes non-homogenous. Moreover, as a result of tinge caused by amorphousness, light absorption and end face heating occur, which consequently leads to accelerated degradation thereof.
Secondly, known nitride-based opto-electronic devices are manufactured on sapphire or silicon-carbide substrates, differing from the thereafter deposited nitride layers (heteroepitaxy). There are significant differences in chemical, physical, crystallographic and electrical properties of such substrates and semiconductor nitride layers deposited thereon by hetero-epitaxy, resulting in rather high dislocation density of the epitaxial semiconductor layers. In order to reduce surface dislocation density and thus to increase stability of the semiconductor laser structures, a buffer layer is first deposited on sapphire or SiC substrates. However, the reduction of surface dislocation density achieved is not higher than to about 108/cm2.
The surface dislocation density could be decreased thus far by using the Epitaxial Lateral Overgrowth (ELOG) method. In this method, a GaN layer is first grown on the sapphire substrate, and then SiO2 is deposited in the form of strips or grids. Next, such a substrate may be used for lateral GaN growing, reducing the defects density to about 106/cm2.
Even further improvement of the substrate for epitaxial formation of nitride semiconductor laser device was attained by a method of manufacturing a bulk monocrystalline layer of gallium-containing nitride disclosed in WO 02/101120.